Field of the Invention
The present invention relates to an image forming apparatus which uses an electrophotographic technique.
Description of the Related Art
Conventionally, an image forming apparatus that forms a color image on a sheet using an electrophotographic technique is known. In such an image forming apparatus, toner images formed in a plurality of process cartridges are primarily transferred to an intermediate transfer belt. Moreover, the toner images primarily transferred to the intermediate transfer belt in a superimposed manner are secondarily transferred to the sheet. Moreover, the toner images secondarily transferred to the sheet are heated and pressed by a fixing apparatus whereby a color image is formed on the sheet.
Here, after the toner images are secondarily transferred from the intermediate transfer belt to the sheet, the toner particles on the intermediate transfer belt may remain thereon. The toner particles remaining on the intermediate transfer belt is referred to as residual toner particles. Moreover, a method of collecting the residual toner particles on the intermediate transfer belt by transferring the residual toner particles back to a photosensitive drum is known. Specifically, the residual toner particles on the intermediate transfer belt are charged by a residual toner charging member. Moreover, the charged residual toner particles are transferred back to the photosensitive drum at a nip portion between the photosensitive drum and the intermediate transfer belt.
Japanese Patent No. 5645870 disclosed a technique related to the residual toner collection method in which the amount of light exposed to a photosensitive drum is decreased to collect toner particles on an intermediate transfer belt. In this way, deterioration of the photosensitive drum is suppressed.
Moreover, Japanese Patent Application Publication No. 2009-205012 discloses a technique related to the residual toner collection method in which the residual toner particles on an intermediate transfer belt are uniformly scattered by a brush member before the residual toner particles are charged by a residual toner charging member. In this way, the residual toner particles on the intermediate transfer belt are uniformly charged by the residual toner charging member. All residual toner particles are transferred back to the photosensitive drum without remaining on the intermediate transfer belt.
Moreover, Japanese Patent Application Publication No. H11-161043 discloses a technique related to the residual toner collection method in which the magnitude of a voltage applied to the residual toner charging member is changed based on temperature and humidity environments. In this way, it is possible to suppress cleaning defects occurring when residual toner particles are not charged sufficiently in a high-temperature and high-humidity environment. Moreover, it is possible to suppress image defects occurring when residual toner particles are charged excessively in a low-temperature and low-humidity environment.
However, in a color image forming apparatus, the thickness (film thickness) of a photosensitive drum may be different among a plurality of process cartridges. When the photosensitive drum is thin, a large amount of current flows to a toner image of an intermediate transfer belt due to a discharge between the photosensitive drum and a primary transfer roller during primary transfer. Due to this, a charge distribution of the toner particles of the toner image on the intermediate transfer belt spreads out. That is, the amount of charges possessed by the individual toner particles of the toner image varies greatly. Toner particles having large positive charges and toner particles having large negative charges are mixed in the toner image. Moreover, the charge distribution of the toner particles on the intermediate transfer belt spreads further during secondary transfer.